GCN Circular 38113

J. Rastinejad, C. Kilpatrick, W. Fong (Northwestern) report on behalf of a larger collaboration:

We observed the location of the X-ray (Kennea et al. GCN 38098
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) and optical (Julakanti et al., GCN 38088
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) counterparts of GRB 241105A (Fermi GBM Team GCN 38085
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, DeLaunay et al. GCN 38091
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, Frederiks et al. GCN 38103
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) with the Gemini Multi-Object Spectrograph (GMOS) mounted on Gemini-South under Program GS-2024B-Q-108 (PI: Fong). We obtained 8x120-sec imaging in r-band at a mid-time of 2024-11-07 01:44:56.1 UT (1.4 days post-burst), at a median airmass of 1.6. 

We clearly detect the optical afterglow. Calibrated to SkyMapper, we measure a preliminary brightness of r~21.7 AB mag at seeing < 1.1'', not corrected for Galactic extinction nor emission from the underlying galaxy (Izzo et al. 38097). Compared to the measurement of r=19.2 +/- 0.2 AB mag at 0.4 days reported by Hu et al. (GCN 38106
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), our observation indicates a rapid rate of decay (Fopt ~ t^-1.8). This decline rate is potentially contaminated by the host galaxy, and the true decline rate may be steeper. While the inferred magnitude differs by ~0.8 mag from the R-band measurement of Dichiara et al. (GCN 38111
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), we deduce similar decline rates in slightly different filters.

Further observations are planned to monitor the variability of the source. We thank Jennifer Andrews and additional Gemini staff for the rapid planning and execution of these observations.